FIG. 1 is a conceptual diagram showing a process of converting the wavelength of a laser pulse, outputted from a common pulse laser oscillator, into half the wavelength using non-linear crystals.
As shown in FIG. 1, the wavelength of a laser pulse outputted from the common pulse laser oscillator (not shown) are converted into half the wavelength through non-linear crystals, such as KTP (KTiOPO4), LBO (LiB3O5), or BBO (BaB2O4). In the example shown in FIG. 1, a wavelength of 1064 nm is converted into a wavelength of 532 nm, which is called frequency doubling or generation of harmonic wave.
A wavelength before being converted is called a fundamental wave. A wavelength reduced by half through conversion is called a second harmonic wave.
Lasers driven by pulses indicate the degree of a change in energy between outputted energies as PTP stability. A decrease in the PTP value indicates a more stable later.
In general, the PTP values of lasers are high when output energy is low, and the PTP values of lasers are low when output energy is high, unless the lasers have special constructions.
The second harmonic wave being generated has a value about twice greater than the PTP value of the fundamental wave and thus has a problem in that the PTP value is increased.
Accordingly, if the fundamental wave of low level energy is incident on non-linear crystals, the second harmonic wave having a very high PTP is obtained because the PTP value of the fundamental wave is high. Accordingly, there is a problem in that the output energy of the second harmonic wave is greatly changed.